The present invention relates to rigid polyurethane resins and composite materials, and more particularly to aircraft radome materials, and the fabrication thereof, which are transparent to high radar frequencies.
Aircraft radomes are generally constructed in a streamlined configuration of composite materials which are particularly transparent to high frequency electromagnetic radiation. Naval aircraft are usually equipped with radomes fabricated from laminates of epoxy resins and glass cloth, but current developments of high performance aircraft and higher frequency radar systems, render these materials unsuitable. The wall thickness of these materials must be increased to withstand the increased airloads and environmental demands such as rain errosion, thermal changes and buffeting, but the increase adversely effects the radome's transparency. Similarly, operation at higher radar frequencies (i.e. shorter wavelengths) also reduces the transparency.
In general radome materials have low dielectric constants and low loss tangents are better for high frequency applications. See von Hippel; Dielectric Materials and Application; MIT Technology Press, 1954. Typical radomes have been fabricated from composites of polyfluorocarbons, polyester and epoxy with glass or quartz fillers, or with laminates of quartz or glass cloth impregnated with these resins. At best, these composites offer a minimum dielectric constant of about 3.25 and loss tangents of 0.015. U.S. Pat. No. 3,577,493 for "Method of Fabricating a Glass Fabric Laminate Material", issued May 4, 1971 to Henry J. Lee, discloses a laminate of polyurethane resin and glass with a low resin-to-glass ratio (20:80) for achieving improved structural properties (flexural strength and elasticity), but it offers no reduction in dielectric constant (4.3). It is theorized that by reducing the polarity of the polymeric molecule of the resin (Daniels et al, Experimental Physical Chemistry, McGraw-Hill, N.Y., 1962; and Pauling, Nature of the Chemical Bond, Cornell University Press, Ithica, N.Y., 1960), and with a proper resin-to-quartz ratio, a dielectric constant of 3.0 or lower is obtainable. The resin synthesis in U.S. Pat. No. 3,577,493 supra utilizes dimers and trimers formed from the 4,4'-MDI (diphenyl methane diisocyanate) monomer. When these polymers are reacted with ethylene glycol, large concentrations of molecules having polar groups, including isocyanates, can form which are electrically dissymmetrical and thereby proportionately increase the dielectric constant. See Ander and Sonnessa, Principals of Chemistry; MacMillan Company; 1965; pages 146-147.